Subsurface contaminants, such as volatile organic compounds (VOC) and semi-volatile organic compounds (SVOC), can be sampled by using an in situ penetrometer probe. One known type of penetrometer probe is a membrane interface probe developed by Geoprobe™. FIG. 1 shows a sampling system 10 of this kind. A probe 30 is advanced, or pushed, through soil by a hydraulic pushing device (not shown) and a set of pipes 29. The probe 30 includes a heating cartridge 34 for heating the soil 35 around the probe 30 and a semi-permeable membrane such as described in U.S. Pat. No. 5,639,956. The membrane prevents exit of carrier gas from the probe 30 to the soil. A transfer line 20 connects apparatus 40 on the surface with the probe 30. The transfer line 20 typically comprises a carrier gas tube 11 for carrying gas in the direction from the surface apparatus 40 to the probe 30, a collection gas tube 12 for carrying gas in the direction from the probe 30 to the surface apparatus 40, and electrical wiring. In use, a carrier gas is delivered from the surface apparatus 40 to an outlet 31 at the probe 30 via tube 11. Contaminants in the heated soil in the region 35 around the probe 30 are collected in the gas at the probe 30, e.g. through the semi-permeable membrane. Contaminant-loaded gas is then conveyed, via tube 12, to detector 42 at the surface. An alternative to the semi-permeable membrane is described in U.S. Pat. No. 6,487,920 with which system, the contaminants in the heated soil in the region 35 around the probe 30 are collected in the gas at the probe 30 directly through an opening and no semi-permeable membrane is used.
A flexible transfer line 20 is required as the line must pass through the pipe sections 29 when they are stacked. Therefore, the line should have a bending radius of approximately 30 cm or less. Also, as the internal diameter of the pipes 22 is approximately 20 mm, there is a restriction for the outer diameter of the transfer line 20.
One of the challenges in transporting the compounds to the surface is to minimise the loss of compounds in the transfer line. These losses in the transfer line can be caused by absorption and adsorption of the compounds at the inner surface of the tube 12 in transfer line 20. Secondly, due to the high moisture level of the collection gas, local moisture condensation in the tubes 11, 12 of the transfer line 20 can obstruct the gas flow, or can increase compound loss through condensation.
Even more important is the cross-contamination between samples which occurs when passing pure product zones (DNAPL's, Dense Non-Aqueous Phase layers, e.g. including tar and chorinated solvents). When passing DNAPL's, compounds are adsorbed on the inner wall of the transfer line. Therefore, the transfer line should be flushed with carrier gas in order to clean the transfer line. Flushing times are typically between 10-60 min which results in long waiting times for the drilling team. This of course is not economically efficient.
In order to minimise the problems relating to adsorption, absorption and condensation, heated transfer lines have been developed. Two methods of heating have been proposed. A first method of heating a transfer line is by wrapping a heating wire around the collection tube. This method suffers from weakness when bending which can result in cold spots and condensation problems. Since this bending capability is crucial when using these tubes for measurements in the field this is a very important prerequisite for the transfer line. A second method of heating a transfer line is shown in U.S. Pat. No. 6,487,920. This uses a silcosteel tube as a collection tube which is resistively heated. Both methods however are very inefficient as a heating method because they require high power, in the kilowatts region, to achieve the required temperatures. Moreover, since the methods are to be used also in remote field conditions where electrical power might not be readily available, additional heavy power generators are necessary. This hampers to a large extent the applicability of the prior art systems.
The methods used in the prior art suffer also from a lack of ease of use as well as safety issues. Because of the high temperatures, the transfer lines are to be extensively thermally insulated to allow manual handling when temperatures significantly higher than 90° C. are required. Also, in U.S. Pat. No. 6,487,920, the line end of the silcosteel tube is at ground potential. This is undesirable as it could result in electrocution, especially in a harsh and humid environment.
There is a need for an alternative form of transfer line which overcomes at least one of the disadvantages of the know transfer lines.